[Illustration: Fig. 121. Left mandible, external side, × 4/1.] MEASUREMENTS SPECIMEN AMEGHINO’S 3091 TYPE Upper dentition, pm. 4 to m. 3 9. mm. 9.5 mm. Upper dentition, pm. 4, length 2.75 mm. Upper dentition, each molar length 2.1 mm. Upper dentition, each molar width 2.25 mm. The lower jaw is slender, the incisor being relatively both smaller and slenderer than in _C. arcidens_; the back part of the ramus light and thin, the coronoid process being a tiny spur, and the articular condyle of small size, and on a level with the teeth. MEASUREMENTS SPECIMEN AMEGHINO’S 3005 TYPE Lower dentition, pm. 4 to m. 3 10 mm. 10.5 mm. Lower dentition, in. 1 to pm. 4 6 mm. 6.5 mm. Lower dentition, pm. 4, length 3.5 mm. Lower dentition, each molar, length 2.5 mm. Lower dentition, each molar, width 2 mm. Height of mandible under pm. 4 5 mm. 4.8 mm. [Illustration: Fig. 122. _C._ plexus, left lower premolar, molar series; _A_, of young individual; _B_, of old individual; _int._, internal side; _ext._, external side, × 4/1.] [Illustration: Fig. 123. _C._ prosus, left upper premolar, molar series, × 4/1.] [Illustration: Fig. 124. _C._ prosus, premolar 4 and molar 2, × 4/1.] =Cephalomys prosus= Ameghino C. prosus Amegh., 1902, Bol. Acad. Nac. Cienc. Cordoba, t. 17, p. 37. This is the tiniest species of the genus, and least frequently found, probably because on account of the small size it was more frequently destroyed before burial, and also because it is hard to find such tiny specimens; so that the sixteen which we found would hardly represent the real proportion of the species in the fauna. The jaws are not only small, but also slender and delicately built, with the premolar about the same size or slightly larger than the molars. The drawings represent the proportions accurately so I will give but a few measurements. SPECIMEN AMEGHINO’S 3009 TYPE Upper premolar 4 to molar 3 8.5 mm. Lower premolar 4 to molar 3 9.5 mm. =Scotamys= gen. nov. A lower jaw, with premolar 4 and molars 1 and 2, from the Deseado beds on the Chico del Chubut River, west of Puerto Visser, indicates a genus of hystricomorph rodents not previously reported. The lower molars suggest those of _Perimys_, but premolar 4 is similar to that of _Scotaeumys_ from the Santa Cruz. _Scotamys_ differs, however, from _Scotaeumys_, in that its molars do not have the third lobe found in the Santa Cruz genus. I have, therefore, made a new genus which appears to be ancestral to _Scotaeumys_. =Scotamys antiquus= sp. nov. [Illustration: Fig. 125. Lower premolar 4—molar 2, × 4/1.]